This invention relates to liquid-filled variable focal length lenses, particularly those used in eyeglasses. Such lenses have been disclosed in a number of prior patents, e.g., U.S. Pat. No. 3,598,479 (Wright 1971), U.S. Pat. No. 5,138,494 (Kurtin 1992) and U.S. Pat. No. 5,668,620 (Kurtin et al. 1997). In particular, this invention is directed to a construction which includes two conceptually distinct parts: 1) a variable focus capsule (which is amenable to volume production since all capsules of a given style can be identical), and 2) a fixed-focus rigid lens, which is attached to the variable focus capsule. The combination results in a lens unit which has the optical power to match a particular person's requirements for all viewing distances--from far to near. The lens units disclosed herein are particularly useful as components of eyeglasses, but it will be appreciated that other applications also exist.
The need for variable focus eyeglasses arises as people age because, generally beginning at about age forty five, the lens in the human eye becomes incapable of sufficient accommodation to focus on near objects. After the onset of this condition of limited focal accommodation, called presbyopia, a single set of fixed focus spectacles will be found to be unsatisfactory for both distant and near vision, irrespective of the wearer's general visual acuity. Whatever correction (if any) may be required to correct a person's vision for distance, an additional amount of optical power (up to about three diopters) will be found to be required to correct that person's eyesight for near vision. The required "near addition" generally does not involve an astigmatic component.
One solution to this problem is to fit presbyopes with spectacles having bifocal (or trifocal) lenses. In the most common form of bifocal lens, the upper part is ground to provide the wearer with the proper correction (if any is needed) for distance vision, and the lower part is ground with the same correction plus an additional amount of optical power to bring near objects into focus. This "near addition" amounts to at most about three diopters of additional optical power. Using bifocal lenses in a pair of spectacles allows a person to see distant objects clearly by looking straight ahead, and to see close objects clearly by looking downward.
Rather than an abrupt change in focal length between the upper and the lower parts of the lens, as in the bi- (or tri-) focal lens, other lens forms have been developed wherein the focal length gradually changes as the eye moves to look through different parts of the lens. These lenses are called "progressive addition lenses." However, progressive lenses inherently suffer from having a limited undistorted field of view. For this and other reasons, only a relatively small fraction of the potential market for multifocal eyeglasses is being met by progressives.
An important difficulty associated with all prior art fixed multifocal spectacles (e.g., bifocals, trifocals and progressives) arises because various parts of the lenses used have different focal lengths. Hence in order to focus on a particular object one must look through the portion of the lens which has the proper focal length for the distance of that object, and this is not always convenient. For example, if one wishes to read the title of a book on an upper shelf, one will find himself looking through the portion of a multifocal lens intended to view distant objects, and the book title will not be in focus.
A liquid-filled variable focus lens avoids this and other problems associated with lenses which provide multiple fixed foci. This is accomplished by providing a lens unit with continuously variable focus, wherein the focal length is substantially constant over the full field of the lens at each setting. The required "near addition" is provided by changing the shape of the lens as needed.
Liquid-filled variable focus lens units for spectacles as described in the prior art (e.g., the aforementioned U.S. patents) involve: 1) a fixed-focus rigid lens, one side of which is shaped to provide the wearer's distance correction, 2) a layer of liquid against the other side of the rigid lens, and 3) a distensible or deformable membrane bounding the side of the liquid away from the rigid lens. If either the volume of liquid between the rigid lens and the membrane is increased (e.g., Wright '479), or the spacing between the rigid lens and the membrane is reduced (e.g., Kurtin '494 and Kurtin et al. '620), the liquid pressure will increase and the membrane will bulge outward so as to increase the optical power of the lens unit. The opposite action will result in membrane relaxation and a decrease in optical power.
Since each potential wearer of eyeglasses requires his or her own prescription, the rigid lens must be ground with a specific wearer in mind. According to the designs of the prior art, the rigid lens forms one boundary of the liquid portion of the lens, and must be incorporated at a very early step in the assembly process. This creates a problem during production in that after the rigid lens is installed, there must be careful tracking of the units throughout the remainder of the manufacturing process to assure that the individual units are identified and not mixed up. Scheduling of production lots may also prove difficult. Hence, with prior art designs, the economies which flow from assembling large quantities of identical products may be difficult to achieve.
It is therefore an object of the present invention to facilitate production of variable focus lens units for eyeglasses (and other applications) by providing a construction which includes a self contained, liquid-filled, variable focus capsule and a separable fixed-focus rigid lens. The variable focus capsule may be fabricated and assembled without reference to the visual acuity of the potential wearer of the eyeglasses, and hence is more amenable to mass production methods.